Organic light-emitting diode (OLED) display

ABSTRACT

An organic light-emitting diode (OLED) display is disclosed. In one aspect, the OLED display includes a plurality of pixels and each pixel includes a first area configured to emit light and a second area configured to transmit external light therethrough. Each pixel also includes a first electrode formed in the first area and an organic layer formed in the first area and the second area, wherein the organic layer covers the first electrode. Each pixel further includes a second electrode covering at least the organic layer formed in the first area and having a first opening exposing at least a portion of the organic layer formed in the second area. A reflection prevention layer is formed substantially covering the organic layer formed in the second area. The reflection prevention layer has a refractive index lower than that of the organic layer.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/469,395,filed Aug. 26, 2014, which claims the benefit of Korean PatentApplication No. 10-2014-0016795, filed on Feb. 13, 2014, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND

1. Field

The described technology generally relates to an organic light-emittingdiode (OLED) display.

2. Description of the Related Technology

Organic light-emitting diode (OLED) displays are exceptional in terms ofviewing angle, contrast, response speeds, power consumption, etc., andthus can be applied to a variety of applications that range from apersonal mobile device such as an MP3 player or a cellular phone to aTV. OLED displays are self-emissive and do not need a separate lightsource in contrast to liquid crystal displays (LCDs). Accordingly, thethickness and weight of OLED displays are comparatively reduced withrespect to displays having a separate light source. OLED displays caninclude a transmission area (or a transmission window) in addition to anarea in which a thin film transistor and/or an OLED are formed. SuchOLED displays function as a transparent display through which externallight can be transmitted.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

One inventive aspect is an organic light-emitting diode (OLED) displaythrough which external light can be transmitted.

Another aspect is an OLED display including a plurality of pixels eachincluding a first area in which light emits and a second area throughwhich external light transmits; a plurality of thin film transistorsformed in the first area of each of the plurality of pixels; a pluralityof first electrodes independently formed in the first area of each ofthe plurality of pixels and respectively connected to the plurality ofthin film transistors; an organic layer provided on the first electrodeand on a position corresponding to the second area; a second electrodecovering the organic layer, connected over the plurality of pixels, andincluding a first transmission window on a position corresponding to thesecond area; a refractive auxiliary layer provided on the secondelectrode on a position corresponding to the first area; and areflective prevention layer provided on the organic layer on a positioncorresponding to the second area and having a lower refractive indexthan that of the organic layer.

The reflective prevention layer may have a lower refractive index thanthat of the refractive auxiliary layer.

The reflective prevention layer may include: a first layer provided onthe organic layer; and a second layer provided on the first layer andhaving a lower refractive index than that of the first layer.

The reflective prevention layer may include irregularities on a surfacethereof.

The refractive auxiliary layer may be formed of 8-quinolionlato lithium.

The reflective prevention layer may include at least one of lithiumfluoride (LiF) and 8-hydroxyquinolatolithum (Liq).

The first layer may include Liq and the second layer may include LiF.

The OLED display may further include: at least one insulating layercovering the plurality of thin film transistors, wherein a secondtransmission window that overlaps the first transmission window isprovided in the at least one insulating layer on a positioncorresponding to the second area.

The first electrode may be provided as a light reflective electrode.

The first area may include a circuit area in which the plurality of thinfilm transistors are formed and an emission area in which the firstelectrode is formed and wherein the circuit area and the emission areaof each of the plurality of pixels overlap each other.

The first transmission window may be independently provided in each ofthe plurality of pixels.

The first transmission window may be connected with respect to at leasttwo of the plurality of pixels that are adjacent to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an OLED displayaccording to an embodiment.

FIGS. 2 and 3 are schematic cross-sectional views illustrating an OLEDdisplay according to embodiments.

FIG. 4 is a detailed cross-sectional view of one of the OLED displays ofFIGS. 1 through 3.

FIG. 5 is a detailed cross-sectional view of the OLED display of FIG. 4according to another embodiment.

FIG. 6 is a schematic plan view of the organic light-emitting unit ofFIG. 4 or 5 according to an embodiment.

FIG. 7 is a schematic plan view of the organic light-emitting unit ofFIG. 6 including an embodiment of the pixel circuit unit.

FIG. 8 is a specific plan view of the pixel circuit unit of FIG. 7.

FIG. 9 is a cross-sectional view taken along line A-A of FIG. 8.

FIGS. 10 through 12 are cross-sectional views of other embodiments takenalong line A-A of FIG. 8.

FIG. 13 is a schematic plan view of the organic light-emitting unit ofFIG. 4 or 5 according to another embodiment.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below, by referring to the figures, toexplain aspects of the present description. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. Expressions such as “at least one of,” whenpreceding a list of elements, modify the entire list of elements and donot modify the individual elements of the list.

FIG. 1 is a schematic cross-sectional view illustrating an organiclight-emitting diode (OLED) display according to an embodiment.

Referring to FIG. 1, the OLED display includes a display unit 2 formedon a substrate 1.

External light is incident onto the OLED display and can be transmittedthrough the substrate 1 and the display unit 2.

The display unit 2 is configured to allow external light to betransmitted therethrough as will be described later. As shown in FIG. 1,the display unit 2 is configured to allow a user at a side where animage is formed to see an image below the substrate 1. Although the OLEDdisplay of the embodiment of FIG. 1 is a top-emission type display inwhich the image of the display unit 2 is emitted in a direction awayfrom the substrate 1, the described technology is not limited thereto.

FIGS. 2 and 3 are schematic cross-sectional views illustrating an OLEDdisplay according to other embodiments.

As shown in FIG. 2, the OLED display can be embodied as abottom-emission type display in which an image of the display unit 2 isemitted in the direction of the substrate 1. As shown in FIG. 3, theOLED display can be embodied as a dual emission type display in whichthe image of the display unit 2 is emitted in the direction of thesubstrate 1 and in the direction away from the substrate 1.

FIGS. 1 through 3 illustrate a first pixel P1 and a second pixel P2 thatare two adjacent pixels of the OLED display. The first and second pixelsP1 and P2 respectively include a first area 31 and a second area 32. Theimage is formed in the first area 31 of the display unit 2. Externallight is transmitted through the second area 32. That is, each of thefirst and second pixels P1 and P2 includes both the first area 31 thatis used to form the image and the second area 32 through which externallight is transmitted so that the environment behind the OLED display canbe seen when a user does not look at the image formed on the displayunit 2.

In this regard, devices such as a thin film transistor, a capacitor, anOLED, etc. are not formed in the second area 32, which results in themaximization of external light transmission. Due to the enhancedtransmissivity of the second area 32, external light through the entiredisplay unit 2 increases, preventing transmitted images from beingdistorted due to interference of the above described devices.

FIG. 4 is a detailed cross-sectional view of one of the OLED displays ofFIGS. 1 through 3. Referring to FIG. 4, the display unit 2 includes anorganic light-emitting unit 21 formed on a first surface 11 of thesubstrate 1 and an encapsulation substrate 23 that encapsulates theorganic light-emitting unit 21.

The encapsulation substrate 23 is formed of a transparent member so thatlight emitted from the organic light-emitting unit 21 can be transmittedtherethrough. The encapsulation substrate 23 can prevent the penetrationof external air or moisture into the organic light-emitting unit 21.Edges of the substrate 1 and the encapsulation substrate 23 areconnected to each other via an encapsulation member or sealant 24 sothat space 25 between the substrate 1 and the encapsulation substrate 23is substantially sealed. The space 25 may include a moisture absorbent,a filling material, or the like.

FIG. 5 is a detailed cross-sectional view of the OLED display of FIG. 4according to another embodiment.

As illustrated in FIG. 5, instead of the encapsulation substrate 23, athin film encapsulation layer 26 is formed on the organic light-emittingunit 21 to protect the organic light-emitting unit 21 from contaminantscontained in the environment. The encapsulation layer 26 may have astructure in which layers formed of an inorganic material such assilicon oxide or silicon nitride and layers formed of an organicmaterial such as epoxy or polyimide are alternately formed. However, thedescribed technology is not limited thereto. Any encapsulation structurecan be used for the encapsulation layer 26.

Although not shown, the encapsulation structure of the organiclight-emitting unit 21 may further include the encapsulation substrate23 of FIG. 4 formed over the encapsulation layer 26 of FIG. 5.

Embodiments of the organic light-emitting unit 21 will now be described.

FIG. 6 is a schematic view of an embodiment including a red pixel Pr, agreen pixel Pg, and a blue pixel Pb of the organic light-emitting unit21. A circuit area 311 and an emission area 312 are included in thefirst area 31 of each of the red, green, and blue pixels Pr, Pg, and Pb.According to the embodiment of FIG. 6, the area of the emission area 312is greater than that of the pixel circuit unit or pixel circuit PC ofthe circuit area 311 so that the pixel circuit unit PC is entirelyhidden by the emission area 312.

The second area 32, through which external light can be transmitted, isadjacent to the first area 31. The second area 32 can be independentlyprovided in each of the red, green, and blue pixels Pr, Pg, and Pb, asshown in FIG. 6.

A plurality of conductive lines such as a scan line S, a data line D,and a power supply line or Vdd line V are electrically connected to thepixel circuit unit PC. Although not shown, various conductive lines maybe provided in addition to the scan, data, and Vdd lines S, D, and Vaccording to the configuration of the pixel circuit unit PC.

FIG. 7 is a specific plan view of an embodiment of the pixel circuitunit PC of FIG. 6.

As shown in FIG. 7, the pixel circuit unit PC includes a first thin filmtransistor TR1 connected to the scan line S and the data line D, asecond thin film transistor TR2 connected to the first thin filmtransistor TR1 and the Vdd line V, and a capacitor Cst connected to thefirst thin film transistor TR1 and the second film transistor TR2. Inthis regard, the first thin film transistor TR1 is a switchingtransistor, and the second film transistor TR2 is a driving transistor.The second thin film transistor TR2 is electrically connected to a firstelectrode 221. Although the first and second thin film transistors TR1and TR2 are P-type transistors in the embodiment of FIG. 7, thedescribed technology is not limited thereto. At least one of the firstand second thin film transistors TR1 and TR3 may be an N-typetransistor. The number of thin film transistors and capacitors is notlimited to those in the illustrated embodiment. Thus, at least two thinfilm transistors and at least one capacitor may be combined in the pixelcircuit unit PC.

Referring to the embodiment of FIG. 7, the scan line S overlaps thefirst electrode 221. However, the described technology is notnecessarily limited thereto. At least one of the conductive lines,including the scan line S, the data line D, and the Vdd line V, may beformed to overlap the first electrode 221. Depending on the embodiment,all of the conductive lines, including the scan line S, the data line D,and the Vdd line V, may be formed to overlap the first electrode 221 ormay be formed adjacent to the first electrode 221.

Due to the separation of the first area 31 and the second area 32,external images can be prevented from being distorted when transmittedthrough the second area 32. Specifically, the scattering of externallight on the patterns of devices included in the pixel circuit unit PCcan be prevented.

The first and second areas 31 and 32 are formed such that the ratio ofthe total area of the first area 31 and the second area 32 to the areaof the second area 32 is within the range of about 5% to about 90%.

If the ratio of the combined areas of the first and second areas 31 and32 to the area of the second area 32 is less than about 5%, when thedisplay unit 2 of FIG. 1 is switched off, the intensity of lighttransmitted through the display unit 2 is low, and thus, a user hasdifficulty seeing an object or an image located on the opposite side ofthe display unit 2. That is, the display unit 2 is not transparent.Although the area of the second area 32 is about 5% of the total area ofthe first and second areas 31 and 32, the first area 31 is formed to bean island with respect to the second area 32, and all availableconductive patterns are formed in the first area 31, which minimizesscattering of external light, and thus, the user may recognize thedisplay unit 2 as a transparent display. When a thin film transistorincluded in the pixel circuit unit PC is formed as a transparent thinfilm transistor such as an oxide semiconductor, and an OLED is formed asa transparent device, the user may further recognize the display unit 2as a transparent display.

If the ratio of the total area of the first and second areas 31 and 32with to the area of the second area 32 is greater than about 90%, thepixel integration of the display unit 2 is extremely low, it isdifficult to stably form an image by emitting light in the first area31. That is, the smaller the area of the first area 31, the higher thebrightness of light required to be emitted from an organic layer 223, asdescribed later, so as to form the image. As described above, if theOLED operates in a high-brightness state, there is a problem that thelifespan of the OLED rapidly declines. If the ratio of the total area ofthe first and second area 31 and 32 to the area of the second area 32 isgreater than about 90% while maintaining an appropriate size of thesingle first area 31, there is a problem that the overall size of thepixel is increased, leading to a low resolution.

However, depending on the embodiment, the ratio of the combined firstand second areas 31 and 32 to the second area 32 can be less than about5% or greater than about 90%.

The ratio of the total area of the first and second areas 31 and 32 tothe area of the second area 32 may be within a range of about 20% toabout 70%.

The area of the first area 31 is considerable greater than that of thesecond area 32 when the ratio of the total area of the first and secondareas 31 and 32 to the area of the second area 32 is equal to or lessthan about 20%, and thus, there is a limit to the brightness of theexternal image the user sees via the second area 32. When the ratio ofthe total area of the first and second areas 31 and 32 to the area ofthe second area 32 is equal to or greater than about 70%, there are manylimitations in designing the pixel circuit unit PC that is to be formedin the first area 31.

The first electrode 221 is electrically connected to the pixel circuitunit PC and is formed in the first area 31. The pixel circuit unit PCoverlaps the first electrode 221 to hide the pixel circuit unit PC. Atleast one of the conductive lines, including the scan line S, the dataline D, and the Vdd line V, may be formed across the first electrode221. In some embodiments, the conductive lines are unlikely tonegatively affect transmissivity compared to the pixel circuit unit PC,and thus, the conductive lines may be formed adjacent to the firstelectrode 221 based on the design requirements. The first electrode 221includes a reflective layer formed of conductive metal that can reflectlight as described later, and thus, the overlapped pixel circuit unit PCis hidden and the external image is prevented from being distorted dueto the pixel circuit unit PC in the first area 31.

Referring to FIGS. 6 and 7, according to an embodiment, a refractiveauxiliary layer 231 is formed in first area 31 and a reflectiveprevention layer or reflection prevention layer 232 is formed in thesecond area 32, thereby increasing light extraction efficiency of theOLED and the transmissivity of external light.

FIG. 8 is a detailed plan view of the organic light-emitting unit 21,according to an embodiment in which the pixel circuit unit PC of FIG. 7is implemented. FIG. 9 is a cross-sectional view taken along line A-A ofFIG. 8.

According to the embodiment of FIGS. 8 and 9, a buffer layer 211 isformed on the first surface 11 of the substrate 1, and the first thinfilm transistor TR1, the capacitor Cst, and the second thin filmtransistor TR2 are formed on the buffer layer 211.

The buffer layer 211 is formed of a transparent insulating material, canprevent the penetration of contaminants, and planarizes the firstsurface 11 of the substrate 1. Thus, the buffer layer 211 may be formedof various materials that are capable of performing these functions. Forexample, the buffer layer 211 may be formed of an inorganic materialsuch as silicon oxide, silicon nitride, silicon oxynitride, aluminumoxide, aluminum nitride, titanium oxide, or titanium nitride; an organicmaterial such as polyimide, polyester, or acryl; or a stack structureincluding the inorganic and organic materials. The buffer layer 211 isnot an essential element and may be omitted according to necessity.

A first semiconductor active layer 212 a and a second semiconductoractive layer 212 b are formed on the buffer layer 211.

The first and second semiconductor active layers 212 a and 212 b may beformed of polycrystalline silicon, but are not limited thereto, and mayalso be formed of an oxide semiconductor. For example, the first andsecond semiconductor active layers 212 a and 212 b may be an I-G-Z-Olayer [(In₂O₃)a(Ga₂O₃)b(ZnO)c layer](where a, b, and c are real numbersthat respectively satisfy the conditions of a≧0, b≧0, and c>0).

A gate insulating layer 213 is formed on the buffer layer 211 to coverthe first and second semiconductor active layers 212 a and 212 b, and afirst gate electrode 214 a and a second gate electrode 214 b are formedon the gate insulating layer 213.

An interlayer insulating layer 215 is formed on the gate insulatinglayer 213 to cover the first and second gate electrodes 214 a and 214 b.A first source electrode 216 a, a first drain electrode 217 a, a secondsource electrode 216 b, and a second drain electrode 217 b are formed onthe interlayer insulating layer 215 so that the first source and drainelectrodes 216 a and 217 a and the second source and drain electrodes216 b and 217 b respectively contact the first semiconductor activelayer 212 a and the second semiconductor active layer 212 b throughcontact holes.

The scan line S may be formed simultaneously with the formation of thefirst and second gate electrodes 214 a and 214 b. The data line D may beformed simultaneously with the formation of the first source electrode216 a so that the data line D is connected to the first source electrode216 a. The Vdd line V may be formed simultaneously with the formation ofthe second source electrode 216 b so that the Vdd line V may beconnected to the second source electrode 216 b.

A lower electrode 220 a of the capacitor Cst may be formedsimultaneously with the formation of the first and second gateelectrodes 214 a and 214 b. An upper electrode 220 b of the capacitorCst may be formed simultaneously with the formation of the first drainelectrode 217 a.

The structures of the first and second thin film transistors TR1 and TR2and the capacitor Cst are not limited to the above description, andvarious structures of thin film transistors and capacitor can beapplied. For example, though the first and second thin film transistorsTR1 and TR2 are illustrated and described as having a top gatestructure, they may also have a bottom gate structure in which the firstand second gate electrodes 214 a and 214 b are respectively formed belowthe first and second semiconductor active layers 212 a and 212 b.

A passivation layer 218 is formed to cover the first thin filmtransistor TR1, the capacitor Cst, and the second thin film transistorTR2. The passivation layer 218 may be an electrically insulating layerincluding a single layer or a plurality of layers that have a planarizedupper surface. The passivation layer 218 may be formed by using aninorganic material and/or an organic material.

As shown in the embodiments of FIGS. 8 and 9, the first electrode 221 isformed on the passivation layer 218 to hide the first thin filmtransistor TR1, the capacitor Cst, and the second thin film transistorTR2. The first electrode 221 is electrically connected to the seconddrain electrode 217 b of the second thin film transistor TR2 through viahole formed in the passivation layer 218. The first electrode 221 may beformed in the form of an island in each pixel, as shown in FIG. 6.

A pixel-defining layer 219 is formed on the passivation layer 109 tocover an edge of the first electrode 221.

The organic layer 223 is formed on the first electrode 221. The organiclayer 223 includes an emission layer (EML) 223 a that emits red, green,or blue light and a common layer 223 b having a monolayer or multilayerstructure that enables the EML 223 a to easily emit light. The commonlayer 223 b may include at least one of a hole injection layer (HIL), ahole transport layer (HTL), an electron transport layer (ETL), and anelectron injection layer (EIL). For example, the organic layer 223 mayhave a structure in which the HIL, the HTL, the EML 223 a, the ETL, andthe EIL are sequentially stacked on the first electrode 221.

The EML 223 a is provided only in the first area 31. That is, the EML223 a may be formed by patterning the EML layer 223 a on the firstelectrode 221 of each of the red, green, and blue pixels. The EML 223 amay include a host material and a dopant material.

The host material may include tris(8-hydroxy-quinolinato)aluminum(Alq3), 9,10-di(naphth-2-yl)anthracene (ADN),2-Tert-butyl-9,10-di(naphth-2-yl)anthracene (TBADN),4,4′-bis(2,2-diphenyl-ethene-1-yl)biphenyl (DPVBi), or4,4′-bis(2,2-di(4-methylphenyl)-ethene-1-yl)biphenyl (p-DMDPVBi).

The dopant material may include DPAVBi(4,4′-bis[4-(di-p-tolylamino)styril]biphenyl), ADN(9,10-di(naph-2-tyl)anthracene), or TBADN(2-tert-butyl-9,10-di(naphth-2-yl)anthracene).

The common layer 223 b may be formed commonly over all pixels. Thus, thecommon layer 223 b may be provided in the second area 32 of each pixelin addition to the first area 31 of each pixel.

The HIL may include a phthalocyanine compound such as copperphthalocyanine or a Starbust type amine such as TCTA, m-MTDATA, orm-MTDAPB.

The HTL may includeN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′-diamine(TPD), or N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (α-NPD).

The ETL may include quinoline derivatives, in particular,tris(8-quinolinolato) aluminum,3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl phenyl)-1,2,4-triazole (TAZ),Balq, and beryllium bis(benzoquinolin-10-olate: Bebq2).

The EIL may include NaCl, CsF, Li₂O, or BaO.

In addition, the materials of, in particular, the ETL or the EILincluded in the common layer 223 b are mostly highly refractivematerials having a refractive index equal to or higher than about 1.8.However, depending on the embodiment, the refractive index of the ETL orthe EIL included in the common layer 223 b can be less than about 1.8.

The second electrode 222 is provided on the organic layer 223. Thesecond electrode 223 is formed over the first area 31 and the secondarea 32.

The first electrode 221 may function as an anode electrode and thesecond electrode 222 may function as a cathode electrode. However, thepolarities of the first and second electrodes 221 and 222 may also beexchanged.

The first electrode 221 may be formed to have a size corresponding tothe first area 31 in each pixel. The second electrode 222 may be formedas a common electrode to cover all pixels of the organic light-emittingunit 21.

According to an embodiment, the first electrode 221 is a reflectiveelectrode and the second electrode 222 is a transparent electrode. Thus,in this embodiment, the organic light-emitting unit 21 is a top emissiontype display in which light is emitted in the direction of the secondelectrode 222.

To this end, the first electrode 221 may include a reflective layerformed of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, or Ca or acompound of these and may include ITO, IZO, ZnO, or In₂O₃ which has ahigh work function. The second electrode 222 may be formed of Ag, Mg,Al, Pt, Pd, Au, Ni, Nd, Jr, Cr, Li, or Ca which has a low work functionor a compound of these. For example, the second electrode 222 may beformed of a Mg:Ag thin film. The second electrode 222 may be formed of athin film to increase transmissivity.

When the first electrode 221 is provided as the reflective electrode asdescribed above, the pixel circuit unit PC formed below the firstelectrode 221 is hidden by the first electrode 221, and thus, as shownin FIG. 8, a user above the second electrode 222 cannot see the patternsof the first thin film transistor TR1, the capacitor Cst, and the secondthin film transistor TR2 that are formed below the first electrode 221.The user also cannot see the portions of the scan line S, the data lineD, and the Vdd line V below the first electrode 221.

When the first electrode 221 is provided as the reflective electrode,light is emitted to a viewer, i.e. in the upward direction, therebyreducing the amount of light that is lost in the direction opposite tothe viewer. As described above, the first electrode 221 functions toconceal the various patterns of the pixel circuit formed below the firstelectrode 221, and thus, the viewer may see a clearer transmissionimage.

The passivation layer 218, the gate insulating layer 213, the interlayerinsulating layer 215, and the pixel-defining layer 219 may be formed astransparent insulating layers.

In addition, a first transmission window or first transmission opening224 is provided on the second electrode 222 in the second area 32. Thefirst transmission window 224 may be formed by removing a portion of thesecond electrode 222 corresponding to the second area 32. The firsttransmission window 224 may be formed in an island pattern. As shown inFIG. 8, the first transmission window 224 may be independently providedin each of the pixels Pr, Pg, and Pb.

FIG. 10 illustrates another embodiment in which a second transmissionwindow or second transmission opening 225 is further formed in thepixel-defining layer 219. The second transmission window 225 may beformed by removing a portion of the pixel-defining layer 219corresponding to the second area 32. The second transmission window 225may be formed to overlap the first transmission window 224. The secondtransmission window 225 may be formed in an island pattern. As shown inFIG. 8, the second transmission window 225 may be independently providedin each of the pixels Pr, Pg, and Pb. Although not shown in FIG. 10, thesecond transmission window 225 may be further formed in at least one ofthe passivation layer 218, the interlayer insulating layer 215, the gateinsulating layer 213, and the buffer layer 211.

According to an embodiment, the first transmission window 224 is formedin the second electrode 222 included in the second area 32, therebypreventing external light from being reflected by the second electrode222 which may be formed of metal. As described above, the reflectivityof external light is reduced, thereby further increasing thetransmissivity of external light in the second area 32.

In addition, a refractive auxiliary layer 231 is provided on a portionof the second electrode 222 corresponding to the first area 31. Therefractive auxiliary layer 231 is patterned and formed in only the firstarea 31 and is not formed in the second area 32. The refractiveauxiliary layer 231 increases the extraction rate of light emitted bythe EML 223 a to improve light extraction efficiency and simultaneouslyprotect the OLED.

The light emitted by the EML 223 a is emitted in all directions. Thelight is partly reflected by the first electrode 221, that is thereflective electrode, and travels in the direction of the secondelectrode 222. The second electrode 222 is a semi-transmission electrodeincluding a thin film metal layer. The light is partially reflected fromthe second electrode 222 to the first electrode 221. As such, the firstelectrode 221 and the second electrode 222 form a microcavity. Theresonance distance may be adjusted by adjusting the distance between thefirst electrode 221 and the second electrode 222, thereby emitting lightof a desired wavelength to the environment. In addition, an air layer300 having a refractive index of 1 is present between the OLED and theencapsulation substrate 23. The greater the refractive index of theinterface between the OLED and the air layer 300, the higher theprobability that the light emitted by the EML 223 a is reflected fromthe interface back to the first electrode 221. The refractive auxiliarylayer 231 having a high refractive index is formed on the secondelectrode 222, and thus, the resonance of the light emitted by the EML223 a can be enhanced, thereby improving the light extraction efficiencyof the OLED.

To increase light extraction efficiency, the refractive auxiliary layer231 is formed of a material having a higher refractive index than thatof the organic layer 223. For example, the refractive auxiliary layer231 may be formed of a material having a refractive index equal to orhigher than about 1.8. For example, the refractive auxiliary layer 231may be formed of 8-quinolionlato lithium. However, this is an example,and the refractive auxiliary layer 231 may be formed of poly(3,4-ethylenedioxythiophene) (poly (3,4-ethylenedioxythiophene), PEDOT),4,4′-bis [N-(3 -methylphenyl)-N-phenylamino] biphenyl (TPD), 4,4′,4″-tris [(3-methylphenyl) phenylamino] triphenylamine (m-MTDATA),1,3,5-tris [N,N-bis (2-methylphenyl)-amino]-benzene (o-MTDAB),1,3,5-tris [N,N-bis (3-methylphenyl)-amino]-benzene (m-MTDAB),1,3,5-tris [N,N-bis (4-methylphenyl)-amino]-benzene (p-MTDAB), 4,4′-bis[N,N-bis (3-methylphenyl)-amino]-diphenylmethane (BPPM), 4,4′-di Yerbaimidazolyl-1,1′-biphenyl (CBP), 4,4′,4″-tris (N-carbazole) triphenylamine (TCTA), 2,2′,2″-(1,3, 3.5-benzene-tolyl)tris-[1-phenyl-1H-benzoimidazole] (TPBI), or3-(4-biphenyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole sol (TAZ).

In addition, the reflective prevention layer 232 is provided on aportion of the common layer 223 b corresponding to the second area 32.The reflective prevention layer 232 is patterned and formed only in thesecond area 32 and is not formed in the first area 31. The reflectiveprevention layer 232 reduces the reflectivity of external light, therebyimproving the transmissivity of light in the second area 32.

External light transmitted through the encapsulation substrate 23 isreflected at the interface between a layer formed on the outermost edgeof the second area 32 and the air layer 300. Although a reflectiveprevention film is formed on an outer surface of the encapsulationsubstrate 23, the reflective prevention film may reduce interfacereflection between the encapsulation substrate 23 and the air layer 300and does not reduce interface reflection between the layer formed on theoutermost edge of the second area 32 and the air layer 300. According toan embodiment, the second electrode 222 is not provided in areacorresponding to the second area 32, and thus, the reflection ofexternal light by metal is removed. However, the common layer 223 b isformed on an uppermost insulating layer corresponding to the second area32. The refractive indexes of various materials forming the common layer223 b are greatly different from that of the air layer 300 having anaverage refractive index of 1, and thus, the reflectivity of the commonlayer 223 b is considerably high. However, when the reflectiveprevention layer 232 having a lower refractive index than that of thecommon layer 223 b is formed on the common layer 223 b, the differencein the refractive index between the reflective prevention layer 232 andthe air layer 300 can be reduced, thereby reducing the interfacereflection.

The reflective prevention layer 232 may be formed of a material having alower refractive index than that of the organic layer 223. For example,the reflective prevention layer 232 may be formed of a material having arefractive index equal to or less than about 1.8. However, thereflective prevention layer 232 may be formed of material having arefractive index of greater than about 1.8. For example, the reflectiveprevention layer 232 may be formed of 8-hydroxyquinolatolithum (Liq).

In this regard, LiF has a refractive index in the range of about 1.3 toabout 1.4, and Liq has a refractive index of about 1.6.

According to another embodiment, the reflective prevention layer 232 mayinclude a plurality of layers, as shown in FIG. 11. For example, thefirst layer 232 a may be formed on the organic layer 223 and the secondlayer 232 b may be formed on the first layer 232 a and contact the airlayer 300. In this embodiment, the second layer 232 b includes amaterial having a lower refractive index than that of the first layer232 a. For example, the first layer 232 a may be formed of Liq and thesecond layer 232 b may be formed of LiF.

The reflective prevention layer 232 includes the plurality of layershaving different refractive indexes as described above, and thus, thedifference in the refractive index between the organic layer 223 and theair layer 300 can be quantitatively reduced. The reflective preventionlayer 232 reduces the difference between refractive indexes at theinterface, which may further reduce the interface reflection between theorganic layer 223 and the air layer 300, and may improve thetransmissivity of light in the second area 32.

TABLE 1 Trans- Reflec- Absorp- missivity tivity tance Classification %(%) (%) Comparative Organic layer/second 86.8 7.1 6.1 example 1electrode/refractive auxiliary layer Comparative Organiclayer/refractive 86.2 13.6 0.2 example 2 auxiliary layer Embodiment 1Organic layer/reflection 93.1 6.7 0.2 prevention layer (FIG. 11)

Table 1 above is a simulation result obtained by the effect oftransmissivity of embodiment 1. In contrast to the comparative examples1 and 2 including the refractive auxiliary layer 231, in embodiment 1including the reflective prevention layer 232 having the structure ofFIG. 11, the transmissivity of external light increases to higher thanabout 90% and the reflectivity of external light dramatically decreases.

Although the reflective prevention layer 232 includes two layers,namely, the first and second layers 223 a and 223 b in FIG. 11, this isan example, and the reflective prevention layer 232 may have a stackstructure of three or more layers.

FIG. 12 illustrates another embodiment that is similar to the embodimentof FIG. 9 and includes irregularities on the surface of the reflectiveprevention layer 232. The irregularities are substantially uniformlyprovided and have a nano-size. Accordingly, in the embodiment of FIG. 12the surface of the reflective prevention layer 232 is uneven. Althoughnot shown, another embodiment is similar to the embodiment of FIG. 11and includes irregularities on the surface of the reflective preventionlayer 232. The irregularities formed in the reflective prevention layer232 scatter external light, and thus, the reflectivity of external lightis reduced. Although not shown, another embodiment includesirregularities on the surface of the reflective prevention layer 232 andis formed of the same material as that of the refractive auxiliary layer231. In this embodiment, the irregularities formed in the reflectiveprevention layer 232 scatter external light, and thus, the reflectivityof external light is reduced.

FIG. 13 is a schematic plan view of the organic light-emitting unit 21according to another embodiment. A single second area 32 is formedcorresponding to the first electrodes 221 a, 221 b, and 221 b of therespective red, green, and, blue pixels Pr. A first data line D1 througha third data line D3 are respectively electrically connected to thefirst electrodes 221 a, 221 b, and 221 c of the red, green, and bluepixels Pr, Pg, and Pb. A first Vdd line V1 is electrically connected tothe first electrodes 221 a and 221 b of the red and green pixels Pr andPg. A second Vdd line V2 is electrically connected to the firstelectrode 221 c of the blue pixel Pb.

Such a structure includes a single large second area 32 with respect tothree pixels, for example, red R, green G, and blue B, thereby furtherincreasing transmissivity and further reducing distortion of an imagedue to scattering of the light. The first transmission window 224 isformed in a position of the second electrode 222 corresponding to thesecond area 32, thereby further improving transmissivity.

Meanwhile, the refractive auxiliary layer 231 is provided in the firstarea 31 and the reflective prevention layer 232 is provided in thesecond area 32, thereby improving the light extraction efficiency of theOLEDs and increasing the transmissivity of external light.

As described above, according to at least one embodiment, thetransmissivity of external light in an OLED display can be improved.

It should be understood that the exemplary embodiments described hereinshould be considered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

While the inventive technology has been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the present invention as definedby the following claims.

What is claimed is:
 1. An organic light-emitting diode (OLED) display,comprising: a plurality of pixels, wherein each pixel includes: a firstarea configured to emit light and a second area configured to transmitexternal light therethrough; a thin film transistor disposed in thefirst area; a first electrode disposed in the first area andelectrically connected to the thin film transistor; an organic emissionlayer disposed on the first electrode in the first area; a secondelectrode covering at least the organic emission layer, wherein thesecond electrode is shared by all of the pixels; a first cappingstructure substantially covering the second electrode; and a secondcapping structure in the second area, wherein the second cappingstructure has a first layer and a second layer disposed directly on thefirst layer and having a lower refractive index than that of the firstlayer.
 2. The OLED display of claim 1, wherein the first layer of thesecond capping structure has a lower refractive index than that of thefirst capping structure.
 3. The OLED display of claim 1, wherein anupper surface of the second capping layer is uneven.
 4. The OLED displayof claim 1, wherein the first layer includes Liq and the second layerincludes LiF.
 5. The OLED display of claim 1, wherein the first cappinglayer includes 8-quinolionlato lithium.
 6. The OLED display of claim 1,further comprising a common layer including at least one of a holeinjection layer, a hole transport layer, an electron transport layer,and an electron injection layer, wherein the common layer is disposedbetween the first electrode and the second electrode in the first areaand in the second area.
 7. The OLED display of claim 6, wherein thesecond electrode has a first opening in the second area and wherein atleast a portion of the common layer disposed in the second area isexposed by the first opening.
 8. The OLED display of claim 7, furthercomprising at least one insulating layer covering the thin filmtransistor, wherein the insulating layer of the pixel includes a secondopening and wherein the second opening at least partially overlaps thefirst opening.
 9. The OLED display of claim 8, wherein the first area ofthe pixel comprises: i) a circuit area in which the thin film transistoris disposed and ii) an emission area in which the first electrode isdisposed, and wherein the circuit area and the emission area at leastpartially overlap each other.
 10. The OLED display of claim 1, whereinthe first electrode is configured to reflect light.
 11. The OLED displayof claim 7, wherein each of the first openings is disposed only in therespective pixel.
 12. The OLED display of claim 7, wherein each of thefirst openings is shared between at least two adjacent pixels.
 13. Anorganic light-emitting diode (OLED) display, comprising: a plurality ofpixels, wherein each pixel includes: a first area configured to emitlight and a second area configured to transmit external lighttherethrough; a thin film transistor disposed in the first area; a firstelectrode disposed in the first area and electrically connected to thethin film transistor; an organic layer including an organic emissionlayer disposed in the first area and a common layer disposed in thefirst area and the second area, wherein the organic layer covers thefirst electrode; a second electrode covering at least the organic layerdisposed in the first area and having a first opening exposing at leasta portion of the organic layer disposed in the second area, wherein thesecond electrode is shared by all of the pixels; a first cappingstructure substantially covering the second electrode; and a secondcapping structure substantially covering the organic layer disposed inthe second area, wherein the second capping structure has a first layerand a second layer provided on the first layer and having a lowerrefractive index than that of the first layer.